1. Field of the Invention
The invention relates to an optical illumination system for illuminating an object and especially to a system for illuminating a spatial light modulator in an image projection or backprojection apparatus. The invention also relates to a projector or a backprojector employing such an illumination system.
2. Related Background of the Invention
In the projection field, it may be necessary to illuminate an individual spatial light modulator using several light sources. This may for example be the case when it is desired to use lamps of limited power levels or when it is desired to use lamps whose light emission spectra are mutually complementary. It is therefore necessary to combine the beams emitted by these sources in order to transmit a single illumination beam onto the modulator.
Various systems such as that disclosed in patent U.S. Pat. No. 6,224,217 (see FIG. 1) are known, in which the beams from two light sources are reflected along the same direction by two reflecting surfaces, such as the external faces of a prism. However, in such a system it is difficult to obtain complete overlap of the two recombined beams so as to obtain a single beam providing uniform illumination.
Other systems, such as that disclosed in patent U.S. Pat. No. 5,504,544 (see FIG. 2), provide an array of prisms that is illuminated by two collimated light beams. The light from each beam penetrates each prism via one face of the prism and is reflected on another face of the prism by internal reflection on this other face. The combination of the prisms of the array thus combines the two beams along a single direction. This system requires a prismatic array that is an expensive component. In addition, the illumination uniformity of the beam obtained as output from the array of prisms is dependent on the uniformity of the beams delivered by the light sources.
Documents U.S. Pat. No. 6,341,876 and US 2001/048562 disclose an optical component for the combining and integrating of beams, which comprises, from upstream to downstream:                upstream, at least one prism, bounded by a first face, called the entrance face, and a second face, which form a dihedron, the entrance face receiving one of the beams coming from the sources, which beam penetrates the prism and is then at least partly reflected by internal reflection on the second face; and        downstream, a light integrator comprising a cylindrical rod of transparent material that receives, at one end, called the entrance end, said beams reflected by at least one-prism, the length of the rod being such that the light of said beams is reflected several times on the inside walls of the rod so as to deliver, at the other end, called the exit end, an almost uniform illumination beam.        
The prisms may be items separate from the integrator rod as in document U.S. Pat. No. 6,341,876, or they may form an integral part of the rod of the integrator system as in document US 2001/048562.
One drawback of such an optical component for combining and integrating beams is that it requires a large number of prisms, namely one per beam to be combined. This becomes even more problematic when the number of beams to be combined is greater than two.
The invention provides an illumination system for solving the aforementioned drawbacks.